This invention relates to a limited rotation motor and method of making same. It relates more specifically to a motor of this type having an integral, variable capacitance displacement transducer.
The motor is designed for limited angular movement, as opposed to ordinary motors capable of an unlimited number of revolutions. It is used in various applications as torque motors for servo valves, meter movements, as well as to drive the marking pens of recorders. In these devices the armature generally rotates against a spring and the amount of rotation is limited by the reaction torque of the spring, which opposes the torque developed by the motor. The motor torque, on the other hand, is a function of the control current applied to the motor.
One motor such as this is described in U.S. Pat. No. Re 26,749. A magnetic armature is mounted for rotation about an axis coaxial with a pair of stationary poles disposed on diametrically opposite sides of the armature. A permanent magnet supplies a static field passing from one pole to the other through the armature. At least one of the poles is split, with a slot extending parallel to the armature axis to divide the pole into a pair of pole pieces. The armature extends across this gap and is thus disposed opposite a part of each of the pole pieces.
Current through a control coil mounted on the stator develops a magnetic field extending from one of the pole pieces into the armature and back to the other of the pole pieces. Thus, depending upon the direction of the current, the control field developed by the coil adds to the static field on one side of the slot and subtracts from it on the other side. The armature is thus subjected to a force in the direction of the stronger field and rotates in that direction about its axis. Generally, the angular displacement of the armature is directly relates to the input current to the motor.
In order to sense the angular displacement of the armature, it is common practice to couple a rotation-sensitive transducer to the motor. The transducer may then be incorporated into a circuit which produces a position feedback signal proportional to armature displacement.
In many cases, the rotation-sensitive transducer is a variable capacitance transducer that is mounted directly on the motor. Typical prior variable capacitance transducers include a pair of spaced-apart stationary plates between which is disposed a movable plate coupled to rotate with the motor armature parallel to the stationary plates. The movable plate may be electrically conductive or non-conductive. In either event, it functions to make the output capacitance of the transducer a sensitive function of armature displacement. Typical transducers of this general type are disclosed, for example, in U.S. Pat. Nos. 3,517,282 and 3,668,672.
In some applications, it is desirable, if not absolutely necessary, that the motor armature diplacement be very uniform for all displacement angles and that the motor have almost unlimited resolution. This, in turn, requires that the motor armature and stator be absolutely concentric and that the armature rotate freely relative to the stator and have no axial displacement. Also, the capacitance of the armature displacement transducer must be related exactly to armature displacement which requirement is hard to achieve with the typical parallel-plate transducers described in the aforementioned patents. This is because it is difficult to mount and maintain spaced-apart plates so that they are exactly parallel to one another, particularly when the motor is being used in areas of high vibration. Also in motors where the armature shaft is used to drive a conductive movable plate, "end play" of the shaft causes unwanted variations in the transducer capacitance as the motor armature rotates.
Also some prior arrangements require slip rings for flexible "pigtail" wires in order to make electrical connections to one or another of the capacitor plates. Accordingly, conventional limited rotation motors with variable capacitance transducers for use in applications requiring a high degree of accuracy and resolution have proven to be difficult to manufacture, at least on an economical basis.